Synthesis of thermoplastic elastomers has generally involved preparation of diblock copolymers having a rubbery segment and a plastic segment at least one of which remains nonterminated and reactive. Subsequently a di- or multifunctional compound is added to the non-terminated diblocks combining at least two of the blocks in a head-to-head orientation to produce a linear triblock composition. The product generally comprises a plastic segment connected to a rubbery segment which is, in turn, connected to a plastic segment. The plastic segment is usually a vinyl aromatic or A block, such as polystyrene, while the rubbery segment is usually a diene or B block, such as polybutadiene or polyisoprene; hence the abbreviation for the linear triblock is ABA.
Preparation via known synthesis of diblocks and the subsequent combining thereof involves anionic polymerization systems, generally organolithium initiated, to form ABLi diblocks which are linked together with a difunctional compound susceptible to lithiun ion attack such as diisocyanates, divinylbenzene, dibromoethane, epoxidized linseed oil or silicon tetrachloride. Examples of such processes are set forth in U.S. Pat. Nos. 3,639,517 and 3,639,521. When a polyfunctional linking compound is employed, such as silicon tetrachloride, a radial or branched structure results wherein four diblocks are linked together. By controlling the amount of selected difunctional linking compounds such as divinylbenzene or diisocynates, a multifunctional nucleus is developed which can also link together a plurality of ABLi diblocks. Such a process is set forth in U.S. Pat. Nos. 3,985,830 and 4,108,945.
While the technique is useful for anionically polymerizable monomers, resort to cationic mechanisms is required for monomers such as isobutylene. Unfortunately, it has not been possible heretofore to prepare thermoplastic elastomers comprising vinyl aromatics and isobutylene. Prior investigations of ours with other co-workers in the laboratories of our Assignee have demonstrated that certain unsaturated organic halides in combination with BCl.sub.3 are effective initiating systems for isobutylene polymerization to form asymmetric telechelic polymers i.e., polyisobutylenes having an olefinic head and an alkyl halide end group. These investigations have been published and include: J. P. Kennedy, S. Y. Huang and S. C. Feinberg, J. Polymer Sci., Polymer Chem. Ed. 15, 2869 (1977); J. P. Kennedy, S. Y. Huang and R. A. Smith, Polymer Bulletin 1,000 (1979); and J. P. Kennedy, S. Y. Huang and R. A. Smith, J. Polymer Sci., Polymer Chem. Ed., (submitted). However, such polymers or B blocks have not successfully been incorporated in ABA triblock copolymers to date.
At least one U.S. patent of which we are aware, is directed toward a method of forming polyisobutylene U.S. Pat. having an unsaturated group at each end. That patent, No. 3,634,383, discloses the contacting of isobutylene with a 5 A molecular sieve supposedly to extract a hydride ion from the monomer. Despite the claim that a product having two terminal double bonds thereby resulted, we are not aware that this work has ever been successfully duplicated and, in fact, the patentee has more recently stated that the polyisobutylene made by 5 A molecular sieves contained less than two olefinic end groups, i.e., 1.70-1.32, per mole. These results have been published: S. L. Manatt, J. D. Ingham and J. A. Miller, Jr., Organic Magnetic Resoanance, 10, 198 (1977).
It has been our belief that asymmetric and symmetric telechelic polyisobutylenes could form the center or B block of various AB and ABA block copolymers. However, we are unaware of any process that would be commercially acceptable for the synthesis of telechelic polyisobutylene and we are equally unaware of the existence of such compositions as homopolymers or as incorporated in thermoplastic elastomer block copolymers. Nor, are we aware of any process for the synthesis of a polymer having at least three functionally terminated polyisobutylene arms or branches which could ultimately each carry an A block, giving rise to a thermoplastic elastomer block copolymer.